Understanding AM Radio Receivers: Antenna Coils & Tuning

Introduction to AM Radio Receivers

This document explains the RF tuning stage in an AM radio receiver, focusing on how it recognizes and processes incoming signals. The tuning stage selects the desired radio station. The resonant frequency of the tank circuit matches the carrier frequency of the station. This applies to commercial AM radio, which operates in the 530 kHz to 1600 kHz range.

We will also describe the characteristics of the antenna coil, including its resistance in the primary and secondary windings, and its design features. Additionally, we will identify the local oscillator (LO) and the mixer.

Recognizing and Identifying the Antenna Coil

An antenna is a device designed to transmit or receive electromagnetic waves in free space. A transmitting antenna converts voltages into electromagnetic waves, while a receiving antenna performs the opposite function. There are many types of antennas, each designed for a specific purpose. In this case, we are dealing with a receiving antenna connected to a tank circuit (coil and capacitor) that modulates the signal induced by a wire (antenna).

Understanding Modulation

Modulation involves adding a signal, called a "carrier," to the original signal. Modulation can be achieved in two primary ways: Amplitude Modulation (AM) and Frequency Modulation (FM).

Amplitude Modulation (AM)

AM involves changing the amplitude of the carrier signal in accordance with the transmitted signal.

Knowing the characteristics of an antenna is crucial for understanding its operation. A receiving antenna influences sensitivity, selectivity, fidelity, and the signal-to-noise ratio.

Important Parameters in AM Receivers:

1) Sensitivity

Sensitivity measures the receiver's ability to capture weak signals. It is determined by the total gain of the receiver (Rx).

2) Selectivity

Selectivity is the receiver's ability to separate adjacent radio stations. It is determined by the bandwidth of the Rx (RF amplifier, converter, IF amplifier).

3) Fidelity

Fidelity is the ability to reproduce the information signal accurately. It depends on the bandwidth of the amplifiers.

4) Signal-to-Noise Ratio

The signal-to-noise ratio is the ratio between the power of the input signal and the noise power output. It measures the strength of the signal relative to the noise.

Construction

Physically, the antenna coil consists of:

Enameled wire is commonly used for coil inductors and transformers. The wire is placed on a paper dielectric, which acts as an insulator. A ferrite core improves the isolation characteristics.